The present invention relates to signal transmitting cables for installation in ducts, and relates particularly, but not exclusively, to optical fibre light signal transmitting cables for installation into underground ducts or into buildings.
It is common to transmit telecommunication signals by means of optical fibre cables installed in underground ducts. In order to minimise the number of persons needed to install such cables, and therefore minimise the installation cost, it is known to install cables consisting of bundles of flexible optical fibres into ducts by means of compressed air. An end of the cable is inserted into one end of a duct, and the cable is then blown into the duct by means of compressed air, fluid drag between the moving air and the surface of the cable causing the cable to move axially along the duct. It is known to provide such cables consisting of optical fibres encapsulated in a thin plastic sheath, which is then encapsulated in a thicker foam plastic layer. The foam plastic layer makes the cable relatively light in weight, but provides it with a large surface area, giving good fluid drag as air passes over it in the installation process.
However, cables of this type, known to persons skilled in the art as MK I cables, suffer from the drawback that because the cable is relatively large in diameter, it is necessary to use a large diameter duct for installation of the cable in order to achieve commercially acceptable installation distances. In the case of telecommunications cables being installed in duct networks, especially in city centre locations where space in underground ducts is scarce because of the very high number of customers and the high cost of construction, it is undesirable to use large ducts. In addition, MK I cables usually use polyethylene foam outer layers, which have relatively poor friction characteristics when in contact with the internal surface of the installation tube, which in turn restricts the blowing distances achievable with this product.
An attempt to solve this problem, known to persons skilled in the art as an MK II cable, is disclosed in International patent application number WO 93/01512, UK Patent application number GB 2282897, and European patent application numbers EP 0422764, EP 0752604 and EP 0345968. In this type of cable, the optical fibres are coated in a two stage process, and the outer layer contains microscopic glass spheres designed to provide a rough surface to enhance fluid drag, while providing a low friction contact surface between the optical fibre cable and the tube. The use of microscopic glass spheres means that the outer layer can be significantly thinner than the foam outer layer of the MK I cable, and the improved friction performance means that this type of prior art cable can be installed over commercially satisfactory distances in smaller diameter ducts.
However, there has persisted a commercial requirement for optical fibre cables which are capable of installation by blowing over longer distances than are possible with the MK II cable, since if the number of optical fibre splices in a cable network can be reduced, the installation time and cost can be reduced and the process can be made suitable for a wider range of applications.
Preferred embodiments of the present invention seek to provide a signal transmitting cable which can be blown into a duct over longer distances than in the prior art.
According to an aspect of the present invention, there is provided a signal transmitting cable for installation in a duct by means of fluid flow, the cable comprising:
a signal transmitting portion comprising at least one elongate, flexible, signal transmitting, member; and
a covering surrounding said signal transmitting portion and having on the outer surface thereof at least two sets of variations in diameter, including a first set of protrusions and/or depressions and a second set of protrusions having respective first and second average amplitudes in a radial direction of the signal transmitting portion and respective first and second average spacing in a respective further second direction along the surface of said covering;
wherein said second average amplitude is greater than said first average amplitude, and said second average spacing is greater than said first average spacing.
By providing a first set of protrusions or depressions and a second set of protrusions, where the average amplitude and spacing of the second set is greater than those of the first set, this provides the highly surprising and advantageous effect of significantly increasing the blowing performance of a signal transmitting cable compared with the prior art cables. It is believed that this effect is achieved by means of the second set of protrusions minimising the area of contact between the signal transmitting cable and the internal surface of the tube (thus minimising friction between the cable and the tube), and the first said of protrusions or depressions at the same time increasing fluid drag between the cable and the compressed fluid blowing the cable into the tube.
At least one said further direction second may be substantially parallel to the axis of said signal transmitting portion.
At least one said further direction may extend helically around said signal transmitting portion.
The first average spacing is preferably arranged such that the air drag imparted to a section of the cable in use during blowing into a tube exceeds
xe2x80x83Aa(p1xe2x88x92p2)Sc/(Sc+Sd), where
Aa is the average cross-sectional area of fluid flowing between the section of the cable and the tube;
(p1-p2) is the fluid pressure difference between the ends of the section of cable;
Sc is the average cross-sectional external perimeter length of the section of cable; and
Sd is the average cross-sectional internal perimeter length of the tube surrounding the section of cable.
In a preferred embodiment, said first average spacing is less than substantially 30 mm.
In a preferred embodiment, the covering comprises at least one coating of material, and said first set of protrusions or depressions is provided on the outermost said coating.
At least one said coating may be of plastics material.
A said first set of protrusions may be printed on the outermost said coating.
In a preferred embodiment, the covering comprises at least one intermittent coating of material, and said second set of protrusions is provided on the outermost said intermittent coating.
At least one said intermittent coating may comprise plastics material.
The second set of protrusions may comprise variations in diameter of the outermost said intermittent coating.
The variations in diameter may be formed by varying the pressure of said plastics material during formation of the outermost said intermittent coating.
The outermost said intermittent coating may be formed by spraying.
In a preferred embodiment, the covering is at least partially formed by extrusion.
The covering may be at least partially formed from crossed textile fibres.
The covering may include fibres of different lateral thicknesses.
The covering may include a mixture of fibres of substantially flattened and substantially circular cross section.
The covering may include fibres of different diameters.
The covering may be braided.
The covering may be woven.
The covering may be knitted.
At least one said fibre may be formed from a plurality of filaments.
In a preferred embodiment, at least one said signal transmitting member is an optical signal transmitting member.
One or more said optical signal transmitting member may be an optical fibre.
At least one said signal transmitting member may be a conductor for transmitting an electrical signal.
The cable preferably further comprises at least one sheath surrounding at least one said signal transmitting member.
The cable may comprise a plurality of said signal transmitting members, and an adhesive layer arranged between at least one pair of adjacent said signal transmitting members.
The second set of protrusions may be provided with a friction reducing coating.
According to another aspect of the invention, there is provided a method of forming a signal transmitting cable as defined above, the method comprising forming said covering around said signal transmitting portion.
The step of forming said covering may include forming at least one coating of material around said signal transmitting portion, and providing said first set of protrusions and/or depressions on the outermost said coating.
A first set of said protrusions may be formed on said outermost coating by means of printing.
The step of providing said second set of protrusions on the outermost said coating may comprise varying the pressure of said material during formation of said coating.
The pressure of said material may be varied by means of a gear pump.
The pressure of said material may be varied by means of at least one solenoid valve.
The step of forming said outermost coating may comprise spraying said outermost coating onto the cable.
The step of forming said covering may comprise at least partially extruding said covering.
The step of forming said covering around said signal transmitting portion may comprise forming said covering of crossed textile fibres.
The step of forming said covering may comprise braiding said textile fibres.
The step of forming said covering may comprise forming said covering from textile fibres of different lateral thicknesses.
The step of forming said covering may comprise forming said covering from textile fibres of different diameters.
The step of forming said covering may comprise forming said covering from a mixture of fibres of substantially flattened and substantially circular cross section.
The method preferably further comprises the step of providing said second set of protrusions with a friction reducing coating.
The first set of protrusions and/or depressions and/or said second set of protrusions may be arranged helically around said covering.
Preferred embodiments of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:-